Wood chips used in the paper and pulp industry are presently produced mainly by means of disc chippers developed for chipping of large amounts of wood. One disc chipper known in the art is described in published application FI 91946. Chipping results show that disc chippers can produce chips of very high quality. With a good chipper the share of the accepted fraction should be about 90%. According to tests a result like this is easy to obtain in test chipping by using a suitable chip length and chipping speed. Even production chippers of big size and with a uniform wood quality and capacity can obtain these values.
However, under normal manufacturing conditions the different factors which the chip quality depends on, like the diameter of the logs, the amount of wood to be chipped and the dry solids content of the wood, vary constantly. The main problems related to chipping by ski means of a disc chipper are in that the amount of sawdust and pin fractions (fine fractions) increases when the amount of oversize and overthick chips (coarse fraction) is reduced. (In the chip size distribution analysing method SCAN-CM 40:94 the chips are distributed into oversize, overthick, accepted, pin and sawdust fractions).
The chippers known in the prior art perform the chipping mostly in the centre of the chipper knives, and the object has been to provide the best chipping conditions in the centre of the knives in order to maximize the quality of the chips. However, the xe2x80x9cscissors forcexe2x80x9d and/or the cutting force the knives moves/move the logs closer to the centre of the disc if the logs are small or dry. Even when the capacity is used to the maximum, the chipping occurs partly in proximity to the centre of the disc and partly in proximity to the outer periphery. In view of the fact that the cutting process is less violent in proximity to the centre, a great deal of coarse fraction is produce in that area. On the outer periphery, instead, a large amount of fine fraction is produced due to the increase in the cutting speed. The cutting force in chipping in a vertical feed chipper and other factors relating to such chippers are described in patent application FI 973078.
The method and the disc chipper according to the invention make it possible to level down these kinds of differences in quality resulting from the chipping in different portions of the disc of the chipper and thus from the chipping at different cutting speeds. The characterising features of the invention are set forth in claims 1 and 4.
The method of producing chips of uniform quality levels out short-term differences in quality resulting from the constantly varying capacity, from the varying diameters of the logs or from the varying dry solids content of the logs. The method produces chips of more uniform quality despite the above-mentioned differences.
The invention and the details thereof will now be described in more detail with reference to the accompanying drawings in which
FIG. 1 shows the disc of a disc chipper and its cutting geometry viewed from the wood feeding side,
FIG. 2 shows the quality of chips as a function of cutting speed,
FIG. 3 is a view of section Axe2x80x94A of FIG. 1, during chipping,
FIG. 4 is a view of section Axe2x80x94A of FIG. 1, in a chipper having a traditional knife equipment,
FIG. 5 shows the quality of chips with different angles a of the front edge of the knife base,
FIG. 6 is a view of section Bxe2x80x94B of the disc of the chipper shown in FIG. 1,
FIG. 7 is a view of a knife strip according to the invention and of sections Cxe2x80x94C and Exe2x80x94E thereof, and
FIG. 8 shows a further method of compensating the cutting speed in the knife base.